Spinal facet fixation device

ABSTRACT

An assembly for the fixation of a spinal facet joint, including an outer shank, an inner shank and a tip, such that rotation of the inner shank with respect to the outer shank causes the tip to move into the outer shank. Movement of the tip causes deformation of the distal end of the outer shank such that the distal end of the outer shank is larger in diameter than the hole through which it was inserted, said deformation causing increased compression and reduced chance of device loosening.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from the provisional patent applicationSer. No. 61/070,795 filed Mar. 25, 2008 in the name of A. Joshua Appeland Marc C. Jacofsky entitled “Spinal Facet Fixation Device”incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a screw for the fixation of the facetjoints of the human spine. More particularly, the invention relates to adevice that is designed for use in the lower thoracic and lumbar spine,but may have wider application in general orthopedic uses includingfracture fixation and implant fixation.

BACKGROUND

The facet joints, or zygapophyseal joints, of the spine are located attwo symmetrical locations at the posterior of the vertebral column. Eachfacet joint consists of two overlapping bony protrusions, the superiorarticular process of one vertebrae and the inferior articular process ofthe neighboring vertebrae. FIG. 18 illustrates two vertebrae as theymate at the facet or zygapophyseal joints.

In certain cases of degeneration of the spinal disk, instability ofvertebral segments, arthritis of the facet joint, or trauma, partial orcomplete immobilization of one or more facet joints is desirable.Traditionally, immobilization is accomplished by anchoring orthopedichardware into the vertebral bodies of adjacent segments, often throughthe pedicle, and interposing a plate or rod between the vertebrae tolimit motion. Additionally, interbody devices are often placed into thedisc space through a variety of techniques to further limit motion andpromote bony fusion between adjacent vertebrae. However, for a number ofreasons, it would be advantageous to eliminate motion and improvestability between two or more vertebrae by directly fastening one orboth of the facet joints together. From a surgical perspective, thefacet joint is easily accessible, thus reducing operative time,decreasing blood loss, decreasing incision size, reducing incidence ofreoperation, and decreasing the risk of potential deleterious effects onnearby anatomic structures, including spinal nerve roots and the spinalcord itself. Further, fixation at the facet joints is morebiomechanically desirable because the center of rotation of the lumbarspine for flexion and extension is located nearest to the facet joints.Thus, placing an immobilization device at or through the facet jointdecreases the torque transmitted through the device, which in turn mayprevent loosening or premature device failure.

In order to provide effective fixation of the facet joint, a fewchallenges must be overcome. Most importantly, a fixation device mustcreate compression between the two articular processes. The compression,which causes or enhances immobilization of the joint by encouragingstability throughout the joint, must be maintained for a significantperiod of time. Additionally, loosening of the device must be prevented.Because the facet joint is generally a mobile joint, forces willcontinue to be transmitted through the joint after the insertion of animmobilization device. Without a specific way to prevent loosening ofthe device, loosening will likely occur as the result of micromotion.Once a device has loosened, the device will often begin to protrude fromor back-out of the bone, causing significant discomfort, damage to thejoint, or danger to surrounding tissues.

Other devices, such as various types of bone screws, have been offeredas ways to fasten the facet joints together. However, each previouslyproposed fixation device has suffered significant shortcomings. Forexample, a standard fully threaded bone screw may be sufficient formerely adjoining two surfaces. However, a fully threaded screw is notcapable of creating any significant amount of compression between twobone surfaces. Any compression generated between surfaces is limited tothe compressive forces generated by the screw threads themselves.Further, there is currently no way to effectively prevent a bone screwfrom loosening over time. When a screw is over-tightened and threads arestripped within the bone, or when threads strip over time as a result ofmicromotion, the compressive force between the facet joint surfaces willbe lost and loosening will likely occur.

To prevent loosening, still other bone screws are designed such that aportion of the screw expands within the bone after the device isimplanted. However, the hoop stress generated by expansion of the devicewithin a bone makes this device ill-suited for use in the relativelysmall bones of the facet joint.

In attempt to create compression and prevent loosening, nut-and-bolttype assemblies have been offered as another method for immobilizationof the facet joint. Using this type of assembly, a screw or “bolt” ispassed through the facet joint and a nut with mating threads is placedaround the screw on the back side of the facet. This approach issuccessful in creating compression and likely at maintaining thecompression over time. However, because the nut must be introduced tothe back side of the facet joint, this approach mandates a procedurethat is significantly more invasive than is otherwise required.

Finally, many devices currently available for fixation of the facetjoint do not contain a central hollow and therefore are not equipped foruse with a guide wire, as is known in the art of orthopedic devices.Without a guide wire, placement of the device within the bones is lessefficient and accuracy is more difficult. Further, small devices, suchas bone screws, can be dropped and even lost within the soft tissuesurrounding the site of insertion.

Because of the shortcomings associated with the currently availablefacet immobilization devices, physicians have largely been hesitant toattempt immobilization of the facet joint, despite the significantbiomechanical and surgical benefits of doing so.

As such, there is a considerable need for a facet fixation device thatcan be easily and effectively inserted through a small incision andextend through the inferior and superior articular processes in order tocreate active compression across the facet joint and limit looseningover time.

SUMMARY OF THE INVENTION

Described below is a cannulated and partially threaded bone screwassembly for the fixation of a spinal facet joint. The assembly isinserted through a very small incision near the facet joint, and has afeature that provides for the expansion of the portion of the deviceprotruding through the superior facet of the inferior vertebrae. Theexpansion feature, along with the partially threaded screw shaft, serveto increase compression between the facet joint surfaces and alsoprevent loosening of the device over time.

In one preferred embodiment the device comprises three parts which canmove with respect to one another. The three parts are an outer screwshank, an inner screw shank, and a tip. The inner shank is disposedwithin the outer shank. The external surface of the outer shank ispartially threaded at the distal end. The external surface of the innershank is partially threaded at the distal end, such that the distal endof the inner shank secures the tip to the screw assembly. The externalsurface of the tip is fully threaded. Additionally, the internal surfaceof the tip is threaded at the proximal end such that the inner shank canscrew into the tip. Once the outer shank is positioned such that itextends through the bones, the outer shank is held stationary and theinner shank is turned. The tip is drawn into the outer shank via thethreads on the inner shank which causes the outer shank to deform. Thedeformation caused by the tip serves to increase the size of the outershank such that it becomes larger in diameter than the hole throughwhich it was inserted. The deformation increases the compressive forceacross the facet joints and also prevents the screw from loosening orbacking out of the hole through which it was inserted.

In another preferred embodiment the inner screw shank and the tip are aunitary piece. Similarly, the inner shank and the tip are disposedwithin the outer shank. The external surface of the outer shank ispartially threaded at the distal end. Additionally, the internal surfaceof the outer shank is partially threaded at the proximal end. Theexternal surface of the inner shank is partially threaded at theproximal end. The external surface of the tip is fully threaded. Oncethe assembly is appropriately positioned through the bones, the outershank is held stationary and the inner shank is turned with respect tothe outer shank. Due to the mating of the threads on the internalsurface of the outer shank and the external surface of the inner shank,the inner shank and the tip are drawn up into the outer shank, causingthe distal end of the outer shank to deform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the cannulated screw assembly, accordingto the first embodiment of the invention.

FIG. 2 is a perspective view of the distal end of the cannulated screwassembly, according to the first embodiment of the present invention.

FIG. 3 is a perspective view of the cannulated screw assembly, accordingto the first embodiment of the present invention, wherein the outershank and tip are shown in solid lines and the inner shank is shownusing hidden lines.

FIG. 4 is a fragmentary side view of the distal end of the cannulatedscrew assembly, according to the first embodiment of the invention,wherein the outer shank and tip are shown in solid lines and the innershank is shown using hidden lines.

FIG. 5 is a perspective view from the distal end of the outer shank,according to the first embodiment of the invention.

FIG. 6 is a perspective view from the proximal end of the outer shank,according to the first embodiment of the present invention.

FIG. 7 is a perspective view of the inner shank, according to the firstembodiment of the present invention.

FIG. 8 is a perspective view of the tip, according to the firstembodiment of the present invention.

FIG. 9 is a further perspective view of the tip, according to the firstembodiment of the present invention.

FIG. 10 is a perspective view of the cannulated screw assembly,according to a different arrangement of the present invention.

FIG. 11 is a cross-sectional view of the cannulated screw assembly,according to the first embodiment of the invention.

FIG. 12 is a perspective view of a cannulated screw assembly, accordingto a second embodiment of the present invention.

FIG. 13 is a perspective view of a cannulated screw assembly, accordingto the second embodiment of the present invention, wherein the outershank and the tip are shown using solid lines and the inner shank isshown using broken lines.

FIG. 14 is a side view of the outer shank, according to the secondembodiment of the present invention.

FIG. 15 is a perspective view of the inner shank and the tip, accordingto the second embodiment of the present invention.

FIG. 16 is a cross-sectional view of the cannulated screw assembly,according to the second embodiment of the present invention.

FIG. 17 is a fragmentary perspective view of a tool used to insert thecannulated screw assembly into bone.

FIG. 18 is a perspective view of a cannulated screw assembly disposedwithin a facet joint, according to an embodiment of the presentinvention.

It should be appreciated that the above description is not meant tolimit the shape of any interface surface and is presented by way ofexample only. For example, a hex-shaped surface of the screw andinsertion tool could be modified to a Torx shape, square shape, or anyother shape that provides an interference fit adequate to rotate thescrew or a portion of the screw assembly. Similarly, threads may vary inlength or be uniform or non-uniform in pitch, and are not limited bytheir depiction in the drawings.

The following Detailed Description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding Background or Summary ofthe Invention or the following Detailed Description of the invention.Reference will now be made in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. The same reference numbers are used throughout the drawings torefer to the same or like parts.

DETAILED DESCRIPTION

Referring generally to FIGS. 1, 2, 3, 4 and 11, there is illustrated afirst embodiment of a cannulated screw assembly 30. The cannulated screwassembly 30 is composed of an outer shank 40, an inner shank 50, and atip 60. The tip 60 and a distal portion of the shaft 41 are threaded tosuch that the device may be inserted into and through two bones. Itshould be generally understood that while in the following descriptionof a preferred embodiment the screw assembly 30 is described as insertedthrough two bones, the device may alternatively be inserted into andthrough two fragments of the same bone or an implantable device and abone.

As shown in FIG. 5, the outer shank 40 comprises a head 43, anunthreaded portion 42 proximal to the head 43, and the threaded portion41 at the distal end of the outer shank 40. The outer shank 40 furtherincludes one or more slits 45 such that the tip 60 can move into thedistal end of the outer shank 40 and cause outward deformation at thedistal end. The outer shank 40 further includes one or more notches 44(shown in FIG. 5) for interaction with one or more projections 61 of thetip 60 (best seen in FIG. 8) such that the tip 60 cannot rotate withrespect to the outer shank 40. Additionally, the outer shank 40 definesa chamfered edge 47, as shown in FIG. 5 and discussed below. As shown inFIG. 7, the outer shank 40 is hollow such that the inner shank 50 can bedisposed within the outer shank 40.

When the screw assembly 30 is fully situated into and through two bones,the unthreaded portion 42 of the screw assembly 30 is disposed withinthe first bone (the bone most proximal to head 43). The threaded portion41 is disposed within and threaded into the second bone. Thisarrangement provides for active compression between the bone surfaces.The length of the screw assembly 30 and the relative length of thethreaded portion of the shank 41 may vary according to the variations inthe anatomy of patients.

Referring now to FIG. 7, the inner shank 50 comprises a head 53, athreaded portion 51 at the distal end of the inner shank 50, and anunthreaded portion 52 proximate to the head 53. The inner shank 50defines an interior cannulated portion 54 through which a guide wire maypass during placement of the assembly 30 within one or more bones.

As shown in FIG. 8, the tip 60 of the cannulated screw assembly 30 isfully threaded 63. The tip 60 also includes a chamfered edge 62, and aself-drilling groove 64. The tip 60 includes the one or more projections61 for interaction with the one or more notches 44, such that the tip 60does not rotate with respect to the outer shank 40. The chamfered edge62 of tip 60 meets the mutually chamfered edge 47 of the outer shank 40where they come together at seam 67 (FIG. 4). The tip 60 further definesa cannulated center 65 for passage of a guide wire during implantationof the device. In a preferred embodiment, the tip 60 is self-drillingand self-tapping, as known in the art.

According to the first embodiment of the invention, and as shown in FIG.9, the tip 60 of the cannulated screw assembly 30 also contains internalthreads 66 at its proximal end to receive the threaded portion 51 at thedistal end of the inner shank 50. When the inner shank 50 is rotatedwith respect to the outer shank 40 the mating of threads of 51 and 66create a force that will cause the tip 60 to be drawn up into the distalend of the outer shank 40. To facilitate the movement of the tip 60 intothe distal end of the outer shank 40, mutually chamfered edges, 62 and47, create a wedge force at the seam 67 as the tip 60 is drawn up by themating of the threads 51 and 66. Additionally, the interaction of one ormore projections 61 and one or more notches 44 prevents the tip 60 fromrotating with respect to the outer shank 40, thus ensuring that theforce between mating threads 51 and 66 serves to move the tip 60 intothe outer shank 40.

According to another embodiment, as shown in FIG. 4, a seating surface46 within the head 43 of the outer shank 40 prevents the head 53 of theinner shank 50 from migrating into the outer shank 40 when a compressiveforce is created by turning the inner shank 50 as to move the tip 60into the outer shank 40. Once the tip 60 begins to move into the outershank 40 the distal end of the outer shank 40 will deform and becomelarger in diameter. To facilitate this enlargement, one or more slits 45are created on the outer shank 40.

Referring next to FIG. 10 there is illustrated a further embodiment ofthe cannulated screw assembly 30. In this embodiment, the head portion43 of the screw assembly 30 includes a collar 75. The collar 75 furtherdefines a securing surface 76. The securing surface 76 may consist ofany surface texture, such as smooth, rough, or ridged (as shown in FIG.10), selected to obtain the desired amount of friction. The collar 75 isdesirable in certain applications in that it provides the surface 76which can be brought into contact with an external bone surface (notshown), in order to distribute forces and loads across the bone. Collars75 of varying dimension can be used. In addition, if desired, anancillary structure such as a washer (not shown) can also be interposedbetween the collar 75 and the bone. A washer can additionally distributeloads across a wider surface of the bone.

According to a second embodiment of the present invention, as shown inFIGS. 12 through 16, the inner shank 50 and the tip 60 are one unitarypiece. The head 43 at the proximal end of the outer shank 40 comprisesan internal threading 49. As discussed with respect to the firstembodiment, the shaft of the outer shank 40 further comprises thethreaded portion 41, and the unthreaded portion 42. Still according tothe second embodiment, and as shown in FIGS. 13 and 15, the inner shank50 includes the unthreaded portion 52, the tip 60, and an additionalexternal threaded portion 55 located on the external surface of theproximal end of the inner shank 50. The tip 60 comprises the outerthreads 63, the self-drilling groove 64, and the cannulated center 65.

According to the above described second embodiment, rotation of theinner shank 50 and tip 60 with respect to the outer shank 40 causes theinternal threading 49 at the proximal end of the outer shank 40 and theexternal threading 55 on the proximal end of the inner shank 50 toengage as shown in FIG. 16. The mating of the threads 49 and 55 createsa force that will cause the inner shank 50 and the tip 60 to be drawn upinto the distal end of the outer shank 40.

The outer shank 40 of the second embodiment, like the outer shank 40 ofthe first embodiment, includes one or more slits 45 to facilitatedeformation, and mutually chamfered tip and outer shank surfaces, 47 and62, which meet at the interface 67 to further facilitate deformation.And similarly, the force associated with drawing the tip 60 up causesthe distal end of the outer shank 40 to deform. The embodiment mayfurther include the previously discussed collar 75 with the securingsurface 76 for distributing forces and loads across the surface of abone.

The foregoing embodiments are preferably comprised of surgical stainlesssteel, titanium, cobalt-chronium alloy, or any other biocompatiblematerial as is known in the art of medical device manufacture. Thecannulated screw assembly 30 may be treated with an adherent layer ofhydroxyapetite, calcium phosphate, or other osteoinductive coatings asknown in the art. Further, the screw assembly 30 may be treated withgrowth factors, stem cells, or any other device coating known in theart, to be selected based on the desired outcome of the procedure.

Referring now to FIG. 17, a custom tool 70 may be used to insert thecannulated screw assembly 30 into and through the bones. The tool 70consists of a male driver device 72, which serves as a provision forengaging and mating with the inner shank, situated within a femaledriver device 73, which serves as a provision for engaging and matingwith the outer shank. In this case they are portrayed as a male andfemale hexagonal-shaped driver, but other shapes are viable options. Thetool 70 is cannulated 71 to allow a guide wire, such as a KIRSCHNER or“K” wire, to pass through. The tool 70 will initially ensure that theinner and outer screw shanks, 40 and 50, rotate at the same rate wheninserting the screw assembly 30 into and through the bones. This willprevent premature and undesired rotation of the inner shank 50 withrespect to the outer shank 40 as the outer shank 40 is placed into bone.Turning the inner shank 50 prior to final placement is undesirablebecause it will cause expansion of the outer shank 40. Once the screwassembly 30 is in the desired orientation, the tool 70 having the maledriver device 72 can be used to rotate the inner shank 50 with respectto the outer shank 40 and thereby cause the desired deformation of theouter shank 40.

The male driver device 72 and the female driver device 73 are preferablycomprised within a single tool 70 including an additional lockingmechanism (shown as a simple set screw 74) such that the user has thecapacity to lock and unlock the ability of the male driver portion 72 torotate with respect to the outer female driver portion 73. The abovedescribed tool is desirable because (i) it ensures that while the outershank 40 is being deformed by the motion of the inner shank 50 and tip60, the outer shank 40 does not turn within the bone, and (ii) iteliminates the added step of changing tools during insertion of thescrew assembly 30. Other equivalent locking and unlocking mechanisms forthis purpose will be apparent to those skilled in the art. For example,the male driver portion 72 and the female driver portion 73 may bemounted on coaxial cannulated stems relatively axially moveable to asmall degree to engage teeth or other interengageable blocking partsinterfitting between portions 72 and 73 to prohibit and permit relativerotation with respect to each other. Less desirable, but still withinthe inventive concept, two different tools may be used—one in which theportions 72 and 73 are fixed with respect to each other for placing thescrew assembly 30, and one in which the portions 72 and 73 are rotatablewith respect to each other to allow the inner shank to be rotated withrespect to the outer shank in order to expand the tip, once the screwassembly is in place. In another alternative and less desirableembodiment, the male driver portion 72 and the female driver portion 73may be two separate tools.

The cannulated screw assembly 30 can be inserted into and through twobones as follows. First, a guide wire, such as a small diameter “K”wire, is inserted into and through the two bones, generally usingfluoroscopic imaging, as is known in the art. Second, the cannulatedscrew assembly 30 is placed over the guide wire. Alternatively, theguide wire may be placed through the screw assembly 30 before the wireis threaded into and through the bones. With the custom tool 70, usingthe self-drilling and self-tapping capacity if necessary, the screwassembly 30 is inserted into and through the bones until the distal tip60 of the screw assembly 30 is protruding through the second bone.Finally, the inner shaft 50 is rotated with respect to the outer shank40 using the custom tool 70, such that the tip 60 is moved up into theouter shank 40 causing deformation of the distal end of the outer shank40. The deformation of the outer shank 40 causes the distal end of theouter shank 40 to permanently expand in diameter outside of and adjacentto the external surface of the more distal bone, thus providingadditional compression between the bones and limiting or preventingloosening over time.

Referring again to FIG. 18, it is noted that neighboring vertebrae comeinto proximity at the facet or zygapophyseal joint 84. The facet joint84 includes a superior articular process 82 of one vertebra and aninferior articular process 83 of a second vertebra. One manner ofconnecting the vertebrae through the zygapophyseal joint using thecannulated screw assembly 30 is illustrated. In the illustratedembodiment, the screw assembly 30 links two proximate vertebrae. Thescrew assembly 30 generally extends beyond the external surface of eachof the bone protrusions that are linked by the assembly 30. In theillustrated embodiment, the screw assembly 30 is positioned with thehead situated in a generally medial orientation. Alternatively, a screwassembly can be positioned with a generally lateral configuration. Inother embodiments, the screw assembly 30 can have a different positionand need not protrude from the bone to the extent illustrated. It isalso to be appreciated that other sections of the same two vertebrae canalso be joined. For example, the second facet joint 84, which ispositioned opposite the first facet joint 84 may be joined by a secondscrew assembly 30. In many circumstances it may be preferable to use thecannulated screw assembly 30 in connection with an interbody device (notshown) in order to achieve maximum immobilization of the spinal section.

The above described embodiments provide significant advantages over thedevices found in the prior art. Specifically, the deformation of thedistal end of the outer shank 40 outside and adjacent to the externalsurface of the bone provides enhanced compression of the bones andprevents the screw assembly 30 from loosening within the bone,preventing discomfort and damage to the joint or surrounding tissues.The partially threaded shaft 41 of the screw assembly 30 further aidesin creating compression and maintaining the compression over time. Thecannula that extends throughout the center of the device allows for useof a guide wire, enabling effective and efficient placement of thedevice. As such, the screw assembly 30 is useful in a variety ofapplications, including fixation of the facet joint, orthopedic fracturefixation, and anchoring an implantable orthopedic device to bone.

While the invention has been described with reference to a preferredembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the invention.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to a particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe general description.

1. A multipart screw assembly for the fixation of two bones or bonesegments, comprising: an outer shank, an inner shank, and a tip; theinner shank and the tip being cannulated; an external surface of theouter shank being partially threaded at the distal end; an externalsurface of the inner shank being partially threaded at the distal end;an external surface of the tip being fully threaded; an internal surfaceof the tip being at least partially threaded at the proximal end suchthat rotation of the inner shank, with its distal end at least partlyreceived within the tip internal surface, causes the tip to move intothe distal end of the outer shank; and wherein the movement of the tipcauses deformation of the distal end of the outer shank.
 2. The assemblyaccording to claim 1, wherein the length of the assembly is greater thanthe combined width of the two bones being joined, and deformation occursoutside of and adjacent to the external surface of a bone.
 3. Theassembly according to claim 2, wherein the tip is self-drilling andself-tapping.
 4. The assembly according to claim 1, wherein the outershank defines one or more slits to facilitate deformation.
 5. Theassembly according to claim 2, wherein the outer shank and the tipcomprise mutually chamfered edges to facilitate deformation.
 6. Theassembly according to claim 5, wherein the outer shank further comprisesa seating surface such that the inner shank cannot move through theouter shank
 7. The assembly according to claim 6, further comprising acollar such that load is dispersed across a surface of a bone.
 8. Theassembly according to claim 1, wherein the tip comprises one or moreprojections and the outer shank comprises one or more notches such thatthe one or more projections interact with the one or more notches,preventing rotation of the tip with respect to the outer shank as theinner shank is rotated.
 9. A multipart screw assembly for fixation oftwo bones or bone segments, comprising: an outer shank, an inner shank,and a tip; the inner shank and the tip being a unitary piece; the innershank and the tip being cannulated; an external surface of the outershank being partially threaded at the distal end; an internal surface ofthe outer shank being partially threaded at the proximal end; anexternal surface of the tip being fully threaded; an external surface ofthe inner shank being partially threaded at the proximal end such thatrotation of the inner shank causes the tip to move into the outer shank;and wherein the movement of the tip causes deformation of the distal endof the outer shank.
 10. The assembly according to claim 9, wherein thelength of the assembly is greater than the combined width of two bonesbeing joined, and the deformation occurs outside of and adjacent to theexternal surface of a bone.
 11. The assembly according to claim 10,wherein the tip is self-drilling and self-tapping.
 12. The assemblyaccording to claim 11, wherein the tip and the distal end of the outershank define mutually chamfered edges.
 13. The assembly according toclaim 12, wherein the outer shank defines one or more slits tofacilitate deformation.
 14. The assembly according to claim 13, furthercomprising a collar such that load is dispersed across the surface ofthe bone.
 15. A method for limiting motion between first and secondbones or bone segments, comprising: inserting a guide wire through thefirst and second bones or bone parts; placing a cannulated screwassembly over the wire, the cannulated screw assembly comprising anouter shank, an inner shank, and a tip; inserting the cannulated screwassembly into the first bone and the second bone until the distal tip ofthe cannulated screw assembly protrudes through the second bone;rotating the inner shaft of the cannulated screw assembly such that thetip is drawn into the outer shank to cause deformation of the distal endof the outer shank adjacent to the external surface of the second bone.16. The method according to claim 15, wherein the step of rotating theinner shaft further comprises causing threads on the internal surface ofthe proximal end of the tip to mate with threads on the external surfaceof the distal end of the inner shank such that the force created byrotating the inner shank draws the tip into the distal end of the outershank.
 17. The method according to claim 15, wherein the tip comprisesone or more projections and the distal end of the outer shank definesone or more notches such that the projections and notches engage andprevent rotation of the tip with respect to the outer shank.
 18. Themethod according to claim 15, wherein the inner shank and the tip are aunitary piece.
 19. The method according to claim 18, wherein the step ofrotating the inner shaft further comprises causing threads on theinternal surface of the proximal end of the outer shank to mate withthreads on the external surface of the proximal end of the inner shanksuch that the force created draws the tip into the distal end of theouter shank.
 20. A method of limiting motion between first and secondbones or bone segments, comprising: providing a cannulated screwassembly comprising an outer shank, an inner shank, and a tip; andfacilitating rotation of the inner shank with respect to the outer shanksuch that the tip moves into the outer shank and causes deformationadjacent to the external surface of the second bone.
 21. The methodaccording to claim 20, wherein the inner shank and the tip are a unitarypiece.
 22. The combination having the multipart screw assembly of claim1 and a screw assembly placement tool, including a stem and a headcarried on the stem; the head having a provision for engaging and matingwith the outer shank for rotating and preventing rotation of the outershank, and a provision for engaging and mating with the inner shank forrotating and preventing rotation of the inner shank; and the provisionsfor engaging and mating with the inner and outer shanks being connectedtogether to rotate together and thereby to rotate the outer and innershanks together during placement of the screw assembly.
 23. Thecombination according to claim 22, further comprising a locking andunlocking mechanism for release of the provision for engaging and matingwith the outer shank from the provision for engaging and mating with theinner shank, to enable rotation of the inner shank independently of theouter shank.
 24. The combination according to claim 23, wherein the stemis cannulated to permit passage of a guide wire through the tool andthrough the screw assembly to guide the screw assembly to its desiredsitus.
 25. A cannulated bone screw assembly placement tool for use witha multipart cannulated screw assembly having an outer shank, an innershank, and a tip, such that when the outer shank and the inner shank arerotated relative to one another, the tip is drawn into a distal end ofthe outer shank for expansion of the outer shank; the placement toolincluding a stem and a head carried on the stem; the head having anouter shank end engagement and mating provision for engaging a proximalend of the outer shank, and for rotating and preventing rotation of theouter shank, and an inner shank end engagement and mating provision forengaging a proximal end of the inner shank, and for rotating andpreventing rotation of the inner shank; and the inner shank and outershank engaging and mating provisions being connected together such thatthe inner shank and outer shank engaging and mating provisions rotatetogether, and are thereby capable of causing the outer and inner shanksto rotate together during placement of the cannulated screw assembly inbone.
 26. The placement tool according to claim 25, further comprising alocking and unlocking mechanism for releasing the outer shank engagementand mating provision from the inner shank engagement and matingprovision, such that the tool is capable of causing rotation of theinner shank independently of the outer shank.
 27. The placement toolaccording to claim 26, wherein the stem is cannulated to permit passageof a guide wire through the tool and through the screw assembly to guidethe screw assembly to its desired situs.